In the manufacturing processes of semiconductor devices, the quality of processes including lithography process, etching process, ion implantation process and the like has great influences upon the yield of the semiconductor devices. Therefore, it is important to detect occurrences of defects and the types thereof at an early stage and feed them back to manufacturing conditions, thereby improving the yield.
In order to inspect circuit patterns formed on a semiconductor wafer in the course of their manufacture, an inspection apparatus in which a scanning electron microscopy is applied (hereinafter, referred to as SEM wafer inspection apparatus) has been employed. The objects to be inspected by this inspection apparatus range widely such as conduction/non-conduction defects, attachment of foreign matters, shape defects of patterns and others.
The SEM wafer inspection apparatus extracts conduction/non-conduction defects by the use of voltage contrasts generated by charging a wafer surface positively or negatively. Herein, the inspection for non-opening defects where a remaining film of an insulator exists at the bottom of a contact hole is taken as an example of the inspection using the voltage contrasts. In the non-opening portion, the remaining film at the bottom of a hole is charged when electron beam is irradiated, and accordingly, a field distribution different from that in the opening portion is formed on the pattern surface. As a result, the number of secondary electrons detected in the opening portion differs from that in the non-opening portion, and these differences are observed as contrasts in an image. In other words, only the defective portions can be detected by comparing the brightness of the contact holes.
In the inspection using the voltage contrasts, it is extremely important to control the charge of the wafer surface. As the method for charging the wafer surface, there are the method using the secondary electron emission efficiency and the method using a control electrode disposed just above a wafer.
In the method using the secondary electron emission efficiency, the polarities of charge are determined by the energy of electrons that enter the wafer and the material of an inspection object. That is, when the secondary electron emission efficiency is 1 or higher, the wafer is charged into a positive polarity, and when it is 1 or lower, the wafer is charged into a negative polarity. In this method, since the charge of the wafer continues until the secondary electron emission efficiency becomes nearly 1, in order to control the charge potential, the incident energy has to be adjusted by the material of patterns.
The method using a control electrode will be described with reference to FIG. 2. In both FIG. 2A and FIG. 2B, only minimum required structural components for describing this method are shown. Acceleration voltage (Va) is applied to an electron source 1, deceleration voltage (Vr) is applied to a wafer 2, and control voltage (Vc) is applied to a control electrode 3, respectively. The electron beam emitted from the electron source 1 is accelerated to the acceleration voltage (Va) and enters the wafer 2 at the energy equivalent to “acceleration voltage (Va)−deceleration voltage (Vr)”. It is assumed here that the energy with which the electron beam enters the wafer 2 at this moment is under the condition that the secondary electron emission efficiency is 1 or higher. The charge of the surface of the wafer 2 is determined by “bias voltage=deceleration voltage (Vr)−control voltage (Vc)”, and FIG. 2A shows the case where the surface of the wafer 2 is charged positively and the condition of the bias voltage>0 and FIG. 2B shows the case where the surface of the wafer 2 is charged negatively and the condition of the bias voltage<0. In the case of FIG. 2A, since the secondary electron emission efficiency is 1 or higher just after the electron beam is irradiated to the wafer 2, the surface of the wafer 2 is charged positively. Then, when the bias voltage and the charge of the surface of the wafer 2 become nearly equal to each other, the number of secondary electrons 33 passing through the control electrode 3 and the number of primary electrons 41 that enter the wafer 2 become nearly equal to each other, and the charge of the surface of the wafer 2 becomes stable. In the case of FIG. 2B, just after the electron beam is irradiated to the wafer 2, many of the secondary electrons 33 emitted from the wafer 2 are returned to the wafer 2 due to the influence of the bias voltage, and the surface of the wafer 2 is charged negatively. Then, when the bias voltage and the charge of the surface of the wafer 2 become nearly equal to each other, the number of secondary electrons 33 passing through the control electrode 3 and the number of primary electrons 41 that enter the wafer 2 become nearly equal to each other, and the charge of the surface of the wafer 2 becomes stable. The advantage of this method lies in that the charge of the wafer surface can be controlled by the bias voltage if the incident energy of the electron beam is under the condition of the secondary electron emission efficiency>1. More specifically, it is possible to control the charge of almost all insulators by an arbitrary incident energy of several 100 to 1000 eV (refer to, for example, Japanese Patent Application Laid-Open Publication No. 11-121561 (Patent Document 1)).